Reliability of boat battery backup w/inverter?

[nickyblase]

I just picked up a 1000A boat battery, with 700w inverter as my disaster contingency plan.

I haven't got the coin to drop a couple grand on a generator, and spent (after discounts) $50 for the battery and $40 for the inverter.

Was this a good idea? The UPS's I've seen out there are expensive (for the good ones).

My goal is to have something available to run my Mak 4 return pump and 2 powerheads in the display (about 10 watts a piece if memory serves). I can't remember the usage of the Mak 4. It's throttled back a little bit, so it shouldn't be using a ton of energy.

I haven't hooked up what I bought yet, in case I get the recommendation from you guys that it was a mistake and I should get a UPS instead.

Thoughts?

 

[Dave McReeferson]

It depends on the inverter, some of them don't generate the correct type of sine wave. What type of inverter did you get?

 

[randoma]

1000A is probably a cranking amp rating - you need to look at amp hours.

Also, if it is a cranking (starting) battery rather than a deep cycle battery, you can only fully discharge it a couple times before it will stop holding a charge.

 

[NateHanson]

Capacity of batteries is measured by Amp Hours, and it's unfortunately unlikely that you've got a 1000AH battery. More likely you've got something around the 100 AH range, unless you and 15 professional wrestlers carried it to your car. Look for a 20 hour capacity rating on the housing of the battery. Also look for what type of battery it is (although for $50, it's likely a flooded battery, probably sealed - other possibilities would be AGM or gel cell.)

I think your setup will do what you want it to, although you'll probably need a trickle charger to keep the battery in good shape. You can get one cheap, and leave it attached to the battery. Without a charger, a flooded battery discharges all by itself quickly enough that over the course of a few months it will lose some charge. And when it sits partially discharged for long periods it can permanently lose some of it's capacity, so you'll want to keep it topped off. (AGM batteries have much lower rates of self-discharge, and may not require a charger)

Your system won't be automatic the way a UPS is, so you'd need to be around when the power goes out to plug your powerheads into the inverter. You could probably set up a relay to act as an automatic transfer switch, but that's beyond me. One of the electrical engineers on here can give you advice on that.

I'd skip the return pump, because your skimmer isn't running anyways, so there's no point of moving all that water through your sump. The return pump uses a lot of electricity, so in the interest of run-time, you'd be much better off just running the powerheads, since it's most important to keep the water in the tank moving, and aerated. (point one of those powerheads up towards the surface!)

To figure out how long your backup battery will run, you need to know the capacity of the battery, and the wattage of your load.

Lets assume your battery is 100 Ah at 12 Volts, and your powerheads are 10 watts each. To figure out the 12V current draw of those powerheads (since your battery is 12V) you divide 20 watts by 12 volts (because P=IV, so I=P/V). So the current draw of the powerheads is 1.7 amps at 12 volts.

If your battery is rated at 100Ah, about half of that capacity is usable without damaging the battery, and before you might drop below the input voltage of your inverter. This is a ballpark thing depending on some factors not clear in this thread, and also assuming that you care about the number of discharge cycles you'll get out of this battery (which you may not because it was cheap, and it's for emergencies). But we'll assume 50Ah usable capacity. Divide that by the 1.7 A that the powerheads are drawing to get the number of hours until you've drawn 50 Ah. The result is 30 hours.

Let's say the mak4 draws something like 100 watts (I think that's close), which is 8.3 amps at 12v. With the powerheads that's a total load of 10 amps, so you'd get about 5 hours running everything. In reality the time would be shorter than 5 hours, because when batteries are discharged quickly their capacity is lower. (that's why the Ah rating is given with a time period, like 20 hours. At 5 hours the Ah rating is quite a bit lower).

So there's all you never wanted to know about lead-acid batteries!

 

[stingythingy45]

Now if you hook that battery and inverter to a lawnmower engine with a pully attached to an alternator.....instant generator.

 

[Jim Tansey]

This goes back a couple of years but when I replaced my boat batteries (not deep cycle) I ran an MJ900 with a 700 Watt inverter in a test with one of the batteries for over 24 hours, not sure how old the battery was. I'm on the Cape and our power outages can last for days, and a major concern is the inverter will not run the heater, being a resistive load I would think it should but it produces little or no heat even when not used with a controller. I guess the waveform produced has a low power factor and the 700 watts is peak.

Jim

 

[Greg Hiller]

I described my back up battery system here:

http://www.reefkeeping.com/issues/2006-05/gh/index.php

 

[randoma]

Greg - do you still recommend the Tsunami AT1 for a pressure activated level controller?

 

[nickyblase]

Hmmm... Good info to have. I will have to take a look at the type of battery and inverter, and swap them out if they are the wrong kind. Maybe I will get lucky, and will have blindly bought the right ones. Somehow, I doubt it, but we'll see!

 

[Jim Tansey]

Nicke,

It does not sound like you purchased anything wrong, just be aware of limitations.

Jim

 

[Greg Hiller]

>do you still recommend the Tsunami AT1 for a pressure activated level controller?<

Yes. Never had one fail. Many in use for more than 5 years.

 

[nickyblase]

Thanks Jim.

I am not sure if the battery is a deep cycle or not, so if it's not, I may try to swap it out, and also get the inverter Greg talks about in his article - it's definitely not the kind I got. Mine doesn't have a trickle charger or a relay.

Depending on cost, that is. If it's going to be a ton more money to get the boat battery thing done in a good way, I may either keep what I have, or try to return them altogether and do the UPS route.

One thing I just thought of though - what about wavemakers? If powerheads are plugged into a wavemaker, would the wavemaker plug into the battery backup (assuming there's a relay)? I wonder how much more wattage that would draw.

Has anyone tried this?

 

[Greg Hiller]

I doubt that a wavemaker pulls much in the way of watts. It's really just a switch.

 

[JayM]

Doesn't the draw of starting and stopping the load on the powerheads lead to an overall higher current draw? Seems it would......